The subunits of b-TSH were separated and purified by an improved two stage gel electrophoresis system. Conditions for reduction of the disulfide structure of b-TSH were reinvestigated to find the optimal conditions for preparation of the subunits. During these studies certain preparations, which had lost most of their biopotency, were found to contain nicked peptide chains which yielded low molecular weight fragments that could be demonstrated by SDS gel electrophoresis.